I think Mckenna made his own position quite clear on several occasions - I could provide links to some relevant lectures if you'd like.
If you're interested in my (less eloquent) distillation I'm happy to oblige.
(BTW sorry if I'm misunderstanding your words. Ambiguity can be assuaged (some what) through clarification - time well spent when discussing unusual topics).
Parsing your question in reverse order -
How would you explain this to a person that thinks this way?
Concerning a stereotypical, utility driven person - I'd agree with foxfire (in that I probably wouldn't attempt to - such an attempt more often than not being futile).
I'd equate 'thinks this way' to 'believes this to be true'. Such beliefs do not arise in a vacuum. Ideologies are usually only renounced when they are seen to fail as an 'in group/out group' strategy - 'Scientism' (to which I believe you are alluding) is alive and well.
Science/would explain this as a disturbance in brain chemistry or psychosis.
The intentional disturbance of brain chemistry plays a huge role in 'everyday' human society. Not just in regards to the ingestion of substances such as legal psycho-actives (caffeine, etc), but especially through activities that provoke the release of neurotransmitters/hormones (having sex, gaining wealth/status, participating in or watching sport, etc).
(Outside of genetic conditions) 'Psychosis' is a qualitative description of human behaviour. It's definition is subjective.
McKenna describes it as vast, dense and alien.
McKenna find significance in visions experienced under the influence of psychedelics.
Our own minds are vast, dense and alien.
Evolution does not just apply to genes. All systems drift without pressure - This includes your own consciousness.
Psychedelics act as an imposition upon the established, repetitive mechanisms that facilitate your own personal evolutionary drift.
A strong psychedelic dose will not just give you an alternate perspective (something many people would benefit from) but will completely
destroy you dissolve your boundaries.
Or even dissolve your boundaries' boundaries...